CPAC A2+3: preparing a standard solution + titration

Question 1

weighing by difference method

Answer 1

• place a weighing bottle on balance then zero
• add mass so you know its the right amount
• remove bottle+mass, zero and reweigh to find total mass, record
• transfer solid to beaker or wherever
• reweigh empty bottle and record
• subtract masses

Question 2

how to prepare a standard solution?

Answer 2

• calculate mass of solid needed
• weigh mass by difference, transfer to rinsed with distilled water beaker
• add distilled water to dissolve. stir with glass rod if needed
• pour solution through funnel and down a glass rod (avoid spillages) into volumetric flask
• rinse beaker, funnel, glass rod. transfer washings to volumetric flask
• make up to the mark using a dropping pipette. read at eye level, bottom of meniscus at the line
• invert flask so its UNIFORM concentration

Question 3

problem with dark liquids

Answer 3

hard to see meniscus

Question 4

what happens if u put hot solutions in flask

Answer 4

• heat causes flask to expand
• volume incorrect

Question 5

why use volumetric flask over measuring cylinder

Answer 5

smaller uncertainty

Question 6

burette rinsed with…

Answer 6

solution inside

Question 7

volumetric pipette rinsed with…

Answer 7

solution inside

Question 8

conical flask rinsed with…

Answer 8

distilled water

Question 9

titration method

Answer 9

• rinse apparatus (see other flashcards)
• 25cm3 UNKNOWN CONC in volumetric pipette
• put into conical flask
• add specified indicator
• place on white tile
• add known conc in burette- jet space filled. record volume
  -add burette solution into conical flask, swirling. add drop wise at end point
• record final volume of burette
• repeat until concordant results

Question 10

concordant results

Answer 10

within 0.2 cm3

Question 11

why concial flask over beaker

Answer 11

easier to swirl without spilling contents

Question 12

why not force out liquid in bottom of pipette

Answer 12

calibration of pippete accounts for it

Question 13

2 consideration with burette

Answer 13

• ensure jet space filled with liquid
• remove funnel

Question 14

problem if jet space not filled properly before titration

Answer 14

larger than expected titre

Question 15

indicators are…

Answer 15

weak acids
- dont add too much

Question 16

phenlphtalein colours

Answer 16

acid: colourless
end point: pink
alkali: dark pink

Question 17

methyl orange indicator

Answer 17

acid: red
end point: orange
alkali: yellow

Question 18

why is it ok to rinse conical flask w water

Answer 18

• doesnt affect moles
• same titre

Question 19

repeatability of concordant results

Answer 19

repeatable

Question 20

why repeat

Answer 20

spot anomolous titres and discard
- concordnant results

Question 21

reducing titration uncertainties

Answer 21

• replace measuring cylinder with more accurate measurer (higher resolution)
• titre a larger volume (increase conc + vol of conical flask)(decrease conc of burette)